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Mortality of fish entering the turbine draft tube (DT) from the tailrace is common during turbine startup or dewatering in Brazilian hydropower dams. Mortality can be reduced if such operations occur when few fish are present in the DT, but estimating abundance therein using traditional gears (e.g., cast net, gillnet, hook-and-line) is constrained by flow and access. We deployed a Dual frequency IDentification SONar (DIDSON) to record and count fish in the DT for the first time using an innovative approach. Methods used elsewhere for manual counting fish in DIDSON images generated dissimilar results. Therefore, we compared three methods of counting fish manually in the frames of DIDSON software from two Brazilian dams: numerical counting (number of fish in the frame), qualitative counting (percent of the frame occupied by fish), and intersection counting (number of intersections of the grid frame with fish). We assumed that the numerical counting provided the best estimates of the true value but could not be used in 32.2% of the frames from one of the dams due to the presence of uncountable aggregation of fish, bubble curtain, or poor contrast between fish and the DT floor. Qualitative counting was the most congruent, whereas intersection counting was the least, based on comparisons with numerical counting. [ABSTRACT FROM AUTHOR]

The recent large-scale hydropower development in the Amazon basin has raised concerns about the impacts on the movements of migratory fishes such as goliath catfish (Brachyplatystoma spp.). In the Madeira River, the efficiency of a 1400-m long fishway in the Santo Antônio hydropower plant (SAE HPP) was evaluated between 2012 and 2016 using telemetry techniques. Tagged fish (N = 388) were released 2000 m downstream of the dam and near the entrance or inside the fishway (N = 149). Fixed radio stations monitored fish movements and residence near 14 different structures of the SAE HPP; four acoustic receivers in the reservoir and one downstream the dam monitored whether fish moved away or past the dam. The findings revealed that 15% of the fish released downstream were detected near the dam but none were detected in the reservoir upstream. For the group of fish released in the fishway, no B. rousseauxii (N = 64) was detected moving upstream and into the reservoir and 5 individuals of B. vaillantii (N = 41) exited the fishway to the reservoir. This was the first long-term telemetry monitoring of fish movements near a dam in the Amazon and the results provided evidence of inefficiency of the fishway for the target species, which led the hydropower company to reconstruct the fishway. [ABSTRACT FROM AUTHOR]

This study identifies hydrodynamic alterations in flow downstream of a dam that are related to hydropower plant (HPP) operation and that might attract Neotropical potamodromous fish to unsafe places in the tailrace during their reproductive migration. Our hypotheses are (1) the hydrodynamic flow in the tailrace presents conditions that are strong attractive for fish than those found in the downstream reach and (2) there are no velocity barriers preventing the upstream migrant from reaching the tailrace over a wide range of turbine discharge. Two calibrated numerical models were created for the Três Marias HPP (Brazil), using different turbine discharges: (1) a three-dimensional (3D) model of flow in the tailrace and (2) a two-dimensional (2D) model for flow in the 3-km reach downstream of the dam. Resulting flow fields of the Três Marias HPP tailrace have elevated hydraulic strain and velocity homogeneity by as compared to the flow field of the downstream reach. The tailrace velocities are slow for mature individuals of all study species, which may swim from downstream reach to unsafe areas in their sustainable and prolonged mode by available corridors for different turbined discharges. [ABSTRACT FROM AUTHOR]

The large catfish Pseudoplatystoma punctifer plays an important role as a top predator in the Amazonian Basin ecosystem. The Madeira River is the largest tributary of the Amazon River marked by a series of rapids and waterfalls. Between 2011 and 2012 some of the rapids were submerged by two hydroelectric dams. Upstream and downstream of the rapids are two sub-basins Mamoré/Guaporé and lower Madeira, respectively. Using mitochondrial and microsatellite markers, we estimated genetic diversity, genetic differentiation, and gene flow between sub-basins and tested if the rapids functioned as barriers limiting the gene flow for this species of catfish. We identified three population clusters, one representing the individuals from the Mamoré/Guaporé sub-basin (upstream) and the other two representing the individuals from the lower Madeira sub-basin (downstream). The rapids restricted but did not block gene flow, which corroborates a clinal pattern of genetic differentiation. [ABSTRACT FROM AUTHOR]

We evaluated the association between the El Niño Southern Oscillation (ENSO), an important climatic phenomenon on Earth, with water level variation and fish assemblage attributes (i.e., abundance, species richness and structure of the assemblage) of the Paraná (dammed) and Ivinhema (undammed) rivers, located in the upper Paraná River floodplain, Brazil. Cross-Correlation Function analyses were performed using the Oceanic Niño Index (ONI; which indexed ENSO events) as the influential time series on water level and fish assemblage attributes time series. ONI were positively associated with water level and with most of the fish assemblage attributes tested, but these patterns differed between the two rivers. This indicates that El Niño events affect the structure of the fish assemblages, mainly in the Ivinhema River, which presented more assemblage attributes associated with ONI. In addition, dams may play a regulatory effect over the water level of the Paraná River, which probably explains the different patterns of assemblage attributes in both rivers. Thus, ENSO events are associated with the composition and structure of the fish assemblages in the studied area, and their effects are apparently minimized in the Paraná River due to the presence of upstream dams. [ABSTRACT FROM AUTHOR]

Despite the socio-environmental importance of ichthyofauna, global fish biodiversity has decreased, and the damming of rivers is one anthropogenic activity that causes this reduction. This scientometric study evaluates the main determinants, directions, and gaps of scientific research on the biodiversity of fishes that are influenced by dams. Between 1991 and 2016, there was an increase in the number of studies about this topic, in proportion to the total number of studies that were evaluated. The distribution of these studies among countries is not random, since some socioeconomic and environmental factors (e.g., gross domestic product, number of researchers, number of large dams) determine the distribution of research. Moreover, these studies mostly address the negative impacts of dams on fish communities on a local scale; several studies have shown the importance of mitigating these impacts. In this particular study, we emphasize the need to research the effects of fishing, cascading dams and the phylogenetic diversity of ichthyofauna. We find that the relationship between dams and fish has become popular among researchers, in particular those who are willing to understand the causes, consequences, and severity of these changes on the ichthyofauna. [ABSTRACT FROM AUTHOR]

Using carbon and nitrogen stable isotope analysis, we characterised food web properties of the deep subtropical Fei-Tsui Reservoir (FTR), which was recently altered from a lotic to a lentic system after dam construction. In the littoral zone, zoobenthos showed strong reliance (83.9%) on benthic algal production. Zoobenthos were never found in the profundal zone because of anoxia. Zooplankton depleted C more than that of particulate organic matter as their putative food source, suggesting a contribution of methane-derived carbon to pelagic food webs. Excluding juveniles, non-native and fluvial species, adult fish showed strong reliance (on average 80.9%) on benthic production, weakly coupled with pelagic food webs. These results contrast low benthic production reliance (on average 27.4%) for a fish community in Lake Biwa, which is also classified as a subtropical lake. Both lakes are characterised by deep pelagic waters but quite different in their geological ages, suggesting that the aquatic communities in the FTR have fluvial origins, and their lacustrine history was too short for them to adapt to newly emerged deep pelagic habitat. Our isotope data are useful as a reference of newly established lentic food webs to monitor ongoing ecological and evolutionary dynamics as a result of anthropogenic disturbances. [ABSTRACT FROM AUTHOR]

Because of their remote location and lack of supporting infrastructure, large glacial rivers of southern Patagonia have remained free of dams. But this is bound to change: two dams proposed for the Santa Cruz River would supply 16% of Argentina's hydropower and five dams planned for the Pascua and Baker Rivers could supply over 20% of Chile's hydropower. In this paper, we project the losses of habitat and juvenile Oncorhynchus mykiss, a recreationally important species, following the construction of the two dams in the Santa Cruz River. We applied a two-stage fitting approach, using generalized additive models and generalized lineal models (GLMs) sequentially to describe habitat-species relationships based on data collected through an intensive field survey of fish and environmental variables along 310 km of river. A simplified GLM trout model based on wetted width, substrate size, and river kilometer was selected which represents the observed geographic distribution very well and with the lowest predictive error. Based on this model, we estimated that habitat supporting 86% of current juvenile trout production will be lost to flooding by the dams. Our data generate a rare opportunity to perform a dam impact assessment by comparison with potential post-dam conditions. [ABSTRACT FROM AUTHOR]

Biological invasions and damming are currently considered the main threats to aquatic biota. This is aggravated by the facilitative effect of damming on dispersal and establishment of non-native species. In this study, the invasion process by Geophagus proximus, an Amazonian Cichlid fish, was examined in different environments of the Upper Paraná River floodplain and Itaipu Reservoir. The effect of sediment retention prompted by damming (increasing water transparency) in the invasion process was investigated, in addition to the possible effects on the phylogenetically related Satanoperca pappaterra. The invasion process of G. proximus began in upstream reservoirs (source of propagules), then this species occupied the floodplain and later dispersed into the Itaipu Reservoir, being most successful (greater abundances) in environments with high water transparency (most above 200 cm). Increased abundances of the invasive species coincide with declined abundances of S. pappaterra. Diet overlap between these species was high, but apparently related to their omnivorous-detritivorous habits. Reproductive strategies were also similar. Therefore, it appears that the main factor that favored the invasive species was the increased transparency resulted from river damming, and that some interaction with S. pappaterra may be occurring. [ABSTRACT FROM AUTHOR]

Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures. [ABSTRACT FROM AUTHOR]

Historical spawning grounds for sea lampreys ( Petromyzon marinus L.) in most Portuguese river basins are becoming inaccessible due to the construction of impassable dams and/or weirs. Studies like the one described in this paper are particularly important in areas like the Vouga river basin, where there is a considerable fishing effort from both professional fishermen and poachers. In fact, for management and conservation purposes, it is important to clarify several aspects of the sea lamprey spawning run in this particular watershed. Therefore, a total of 30 radio tagged, migrating sea lampreys were released in the River Vouga and some of its main tributaries during 2004 and 2005. Results from the tracking sessions were used to determine the effect of poaching on the spawners’ population and the characteristics of the resting sites used during the upstream movement. The rivers’ stretches were also characterized according to the type of substrate present in the riverbed and flow type, in order to determine its aptitude to constitute appropriate spawning habitats for sea lampreys. We have identified in the upstream stretches of River Vouga, and in the tributary River Caima, characteristics that are particularly suitable for the construction of nests by the spawners. Presence of larvae provided evidence that spawners migrated into the River Vouga’s upper reaches. However, abundance and age class diversity appeared to be higher downstream of Sernada and Carvoeiro weirs, corroborating the telemetry data which suggested difficulty in passing these obstacles during low precipitation years. In the River Caima, migration was most predictable, which might be related to daily water releases from a small hydropower dam at dusk, that stimulated the lampreys to resume migration. Poaching has had a considerable negative effect on the success of the tagged lampreys’ migration: 76% of the animals released during 2005 were captured. [ABSTRACT FROM AUTHOR]

The autumn and spring descent of 41 brown trout kelts (spent trout) (average total length Lt 75.9 cm) and the spring descent of 27 hatchery-reared smolts (average Lt 26.2 cm) were studied by radiotelemetry in 1993/1994 (kelts) and 1997/1998 (kelts and smolts) at the Hunderfossen dam and power plant, south-east Norway. In 1999 we studied spring descent of 48 untagged kelts by visual observations and video-monitoring at one spillway. In autumn 1993 and 1997, 62.4% and 44.0% of the tagged kelts migrated downstream to the dam at water temperatures between 0.1 and 0.8°C. During release of spillwater, the kelts gathered along the dam with limited movements. Neither smolts nor kelts used the 2 m submerged turbine shafts as a pathway to migrate downstream Hunderfossen dam. Nor did release of large amounts of deep water through spillways provide downstream migration possibilities for kelts and possibly for smolts. The majority of both smolts and kelts migrated downstream at short periods of surface water release through the spillways, indicating high importance of surface water release. The threshold value of descend of kelts at surface release was between 1 and 4 m3 s−1 which correspond to a water column between 12 and 36 cm. These findings are highly relevant regarding hydroelectric development in river systems containing iteroparous salmonid species. [ABSTRACT FROM AUTHOR]